The present invention generally relates to cardiac arrhythmia detectors, and more particularly, concerns a method and apparatus for determining and declaring the presence of ventricular fibrillation in an electrocardiograph signal which is being monitored.
In patients having cardiac problems or surgery, one of the major concerns is the potential for the onset of ventricular fibrillation. For whatever the reasons may be for causing ventricular fibrillation, its occurrence is a most serious event. The quivering of the ventricles associated with ventricular fibrillation causes cessation of the cardiac output. In a high percentage of patients ventricular fibrillation represents the terminal cardiac event. Although ventricular fibrillation is quite serious, it can be controlled or arrested by heart massage and defibrillation techniques. One well known defibrillation technique is the application of a severe electrical shock to the patient's heart which subdues the excitation of the ventricles. However, the defibrillation application must be performed quickly in order to overcome the cessation of cardiac output. This time period for the application of defibrillation must be within a few minutes from the onset of ventricular fibrillation. Therefore, monitoring the patient for purposes of detecting ventricular fibrillation is an important procedure involving cardiac patients.
Conventional bedside electrocardiograph (ECG) monitors in general are not adequate for reliable ventricular fibrillation detection. For instance, many heart rate meters are optimized to detect well defined QRS complexes, and may detect ventricular fibrillation waveforms as beats only sporadically, causing either a long delay before providing an alarm, or even worse, no alarm indication at all. Furthermore, ventricular fibrillation rates may be as low as one hundred twenty (120) beats per minute and may be below the upper heart rate alarm limit. Meters that reject T-waves pose problems because one half of the ventricular fibrillation waveform may be classified as a T-wave. Even some computerized arrhythmia monitoring systems have difficulty in providing reliable ventricular fibrillation detection if they employ noise detectors that do not distinguish a ventricular fibrillation period from a high noise period.
A number of apparatuses for detecting ventricular fibrillation by monitoring ECG waveforms have been proposed. Various types of these apparatuses have been described, for example, in U.S. Pat. Nos. 3,612,041 and 3,598,110. In the former patent, fibrillation is indicated when the signal occurs at a two hundred (200) to five hundred (500) beat per minute rate. As indicated above, ventricular fibrillation rates may be as low as one hundred twenty (120) beats per minute. In this instance, such ventricular fibrillation will be below the indication level relied upon by U.S. Pat. No. 3,612,041. In the latter patent mentioned above, the circuit seeks to establish permature ventricular contraction rather than ventricular fibrillation. Also, the circuitry is somewhat complex in nature. Another apparatus for monitoring recurrent waveforms to establish, for example, ventricular fibrillation, is disclosed in U.S. Pat. No. 3,654,916. However, in that patent, the abnormality of waveform complexes is relied upon to establish ventricular fibrillation as being present in the waveform. Various complicated steps to produce a voltage level related to the mean irregularity of shape of the ECG complex are required to establish that ventricular fibrillation is present.
Another more recent technique for detecting ventricular fibrillation has been proposed and involves the use of fast Fourier transforms (FFT's). An explanation of this technique is found in two recently published periodicals, Nygards, M. E., and Hulting, J., "Recognition of Ventricular Fibrillation Utilizing the Power Spectrum of the ECG," Computers in Cardiology, pages 393 to 397, IEEE Computer Society, 1977, and Kuo, S. and Dillman, R., "Computer Detection of Ventricular Fibrillation," Computers in Cardiology, pages 347 to 349, IEEE Computer Society, 1978. The basic notion associated with the FFT technique is to equate ventricular fibrillation with the presence of a sinusoidal signal; this is accomplished by examining the spectrum of the incoming ECG signal, determining its mean frequency and bandwith, and if these two measurements satisfy certain constraints, then ventricular fibrillation is declared to exist. While this technique appears to be quite accurate, it also has a number of deficiencies or limitations. For instance, the equipment and approach used in the FFT technique trade computational complexity for enhanced accuracy. The minimum computational cost for the known FFT technique has been estimated at 2Nlog.sub.2 N multiplies, the requirement for buffering N data words (the number of data points to be analyzed), and the complexity of programming FFT equipment. This computational cost in this large number of real multiplies does not include the determination of bandwith and mean frequency of the signal. As a result, these proposed FFT techniques employ large, complex computations that are generally impractical to be included in bedside cardiac monitors. Accordingly, it can be seen that the shortcomings in equipment and techniques to detect and declare the presence of ventricular fibrillation require improvements in this most significant component in the monitoring of cardiac patients.